A number of elastomeric polymers, such as polybutadiene, natural rubber, styrene butadiene rubber, and isoprene rubber, have been used as the base polymer in the manufacture of golf ball cores. Today, golf ball cores are predominantly made from compositions comprising polybutadiene, which provides the primary source of resiliency for the golf ball. Further increase in core resiliency can be achieved by increasing the cross-link density of the polybutadiene. A peroxide-based free radical initiator such as dicumyl peroxide is typically added in conjunction with a crosslinking agent such as zinc diacrylate and/or zinc dimethacrylate to crosslink polybutadiene and obtain the desired physical properties for the golf ball cores. Zinc oxide is also added to the composition, both as a density-modifying filler and as an activation agent for the zinc diacrylate/peroxide curing system. The core is protected by a cover, and may comprise additional layers in between, such as outer core layers, intermediate layers, or inner cover layers. Any one of these additional layers may be a wound layer of tensioned elastic windings.
A crosslinking agent is fully effective only if it is thoroughly and homogeneously dispersed throughout the base polymer. At the relatively high mixing temperatures required for the base polymer, many organic peroxides become thermally unstable, making it impossible to achieve a homogeneous dispersion of the crosslinking agent in the base polymer. Best results have been obtained with dialkyl and diaralkyl peroxides, for instance di-t-butyl peroxide (“DTBP”) and dicumylperoxide (“DCP”), but not without shortcomings. DTBP is so volatile that its use is very much limited. DCP can be used under normal operating conditions only at the cost of considerable environmental pollution since its principal decomposition product is acetophenone, which imparts to the finished product and to the environment a very intensive unpleasant odor.
Therefore, a need remains for a golf ball core composition that minimizes the above-described problems and provides a golf ball core with improved characteristics, such as compression, initial velocity, durability and resiliency. The present invention provides such core compositions for golf balls.